drm/i915: Prepare for larger CSB status FIFO size

Make csb entry count variable in preparation for larger
CSB status FIFO size found on gen11+ hardware.

v2: adapt to hwsp access only (Chris)
    non continuous mmio (Daniele)
v3: entries (Chris), fix macro for checkpatch
v4: num_entries (Chris)
v5: consistency on num_entries

Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405204657.12887-1-chris@chris-wilson.co.uk